Surgical devices that deploy an electrical circuit between electrodes do so in an electrically conductive medium, which may be either in vivo biologic tissues or delivered media such as electrolyte solutions. The tissue effects produced by these devices are dependent upon the events occurring at or around the electrodes as electrical energy is converted to therapeutically useful forms. Converted energy forms can be either near-field at the electrode surface or far-field projected away from the electrodes. Near-field effects are produced by electrical current and include physiochemical events like electrothermal and electrochemical conversions; far-field effects are produced by electromagnetic radiation forces like magnetic flux densities, voltage potentials, or displacement currents generated around the electrodes. Gross electrical conduction in biological tissues is principally due to the conductivity of in situ interstitial fluids which are electrolyte water-based and thus predominantly ionic. Since the electrical charge carriers in metal electrodes are primarily electrons, the transition between electronic and ionic conduction is governed by physiochemical processes at the electrode-to-fluid interface within the conductive media, even though this process can be altered by electrode contact with macromolecular biologic material. Electrically conductive media solutions have been used for many decades to complete surgical device circuits and no longer alone serve as a proprietary method of circuit completion (Elässer E, Roos E. Uber ein instrument zur leckstromfreien transurethralen resektion [An instrument for 440 transurethral resection without leakage of currents]. Acta Medico Technica. 1976; 24(4):129-134). Both direct current and alternating current formulations have been deployed in surgical device designs.
Surgical use of direct current induces tissue necrosis as a means to destroy unwanted tissue through near-field electrical current effects delivered into biologic structures. Electrolytic ablation, or tissue electrolysis, is a technique which consists of placing an anode electrode and cathode electrode at various points within or adjacent to tissue and driving direct current (40-100 mA) between them and through the biologic mass to induce tissue electrolysis. The products of tissue electrolysis kill cells by creating, in a spherical area surrounding the each electrode, local changes in tissue pH too large for cells to survive. These pH changes are caused by creating toxic products such as chlorine, oxygen, and hydrogen ions at the anode electrode and hydrogen gas and sodium hydroxide at the cathode electrode. The region surrounding the anode becomes very acidic (˜pH 2) and surrounding the cathode becomes strongly alkaline (˜pH 12) with the amount of necrosis dependent upon the total electrolysis dose measured in coulombs as a product of tissue current delivery and time. A pH less than 6.0 at the anode and greater than 9.0 at the cathode reflects total cellular necrosis. Direct current applications deliver static electromagnetic fields that have inconsequential quanta in the regions of non-necrotic tissue. Electrolytic ablation does not rely upon a thermal effect as tissue temperatures only rise minimally during these procedures to levels not associated with cell death.
Surgical use of alternating current has been designed to induce therapeutic necrosis for volumetric tissue removal, coagulation, or dissection through near-field electrical current effects within biologic tissues. Radiofrequency wavelengths and frequencies do not directly stimulate nerve or muscle tissue; and, so are prevalent in medical applications. Radiofrequency surgical devices utilize tissue as the primary medium like in direct current applications; however, these surgical devices produce resistive tissue heating (ohmic or Joule heating) by an alternating current induced increase in molecular kinetic or vibrational energy to create thermal necrosis. In order to obtain the desired levels of thermal necrosis through resistive heating in a media with exceptionally large specific heat capacity such as water found in and around biologic tissues, high-levels of alternating current deposition are required to maintain heat production, and conduction to remote tissue, in the presence of treatment site thermal convection. In certain settings, high-level energy radiofrequency devices can be configured to produce water vapor preferentially through very rapid and intense resistive heating, overcoming the high heat of vaporization at the treatment site. Coincident with this method, the far-field time-varying electromagnetic forces of these devices deliver quanta able to generate charged plasma particles within the water vapor cloud. This ionizing electromagnetic radiation can induce an electron cascade, which operates over very short distances (Debye sphere) and with electron temperatures of several thousand degrees Celsius, to produce therapeutic molecular disintegration of biologic tissues as its action decays into heat. Radiofrequency thermal ablation and plasma-based techniques display use limitations associated with their design. Thermal and plasma lesions spread according to induced gradients; but, because of the variable energy transfer coefficients in the treatment settings of biologic tissues, iatrogenic tissue charring, necrosis, and collateral damage from imprecise heating or excess energy deposition can occur.
Electrolytic ablation, radiofrequency thermal ablation, and radiofrequency plasma-based surgical devices are designed for a direct electrode-to-tissue interface, concentrating near-field electrical energy to perform surgical work centered upon therapeutic necrosis. In these applications, collateral damage is a normal procedural consequence since the application locales to which these devices are deployed can often accommodate an excess or imprecise application of energy to ensure expedient procedural efficacy within varying treatment site conditions.
From a surgical work energy procurement standpoint, these procedures are defined by an inefficient use of electrical energy due to the excess energy deposition that occurs within biologic tissue producing iatrogenic collateral damage. Far-field electromagnetic forces, although present, are confounded by tissue current deposition or, in the case of plasma-based radiofrequency devices, are of such a high intensity constituting local ionizing electromagnetic radiation. Electrolytic ablation, radiofrequency thermal ablation, and radiofrequency plasma devices all struggle in balancing volumetric tissue removal with healthy tissue loss because of excess collateral energy deposition into the tissue.